Most girls get their first period around age 12, but anywhere from 9 to 16 is within the normal range. If you’re wondering why yours hasn’t arrived yet, or why it stopped after starting, the answer usually comes down to where your body is in its development, your weight and nutrition, your stress levels, or a hormonal imbalance that’s treatable once identified.
What Counts as “Normal” Timing
The average age of a first period has been shifting slightly earlier over the past few decades. For people born between 2000 and 2005, the average age dropped to 11.9 years, compared with 12.5 years for those born in the 1950s and 1960s, according to a large study from Harvard. A first period before age 9 is considered very early, and one that hasn’t arrived by age 16 is classified as late (called primary amenorrhea). Everything in between is typical.
A useful marker: periods usually start about two years after breasts begin to develop or pubic hair begins to grow. If you’ve noticed those changes within the last year or two, your body may simply not be there yet. If breast development started more than three years ago and you still haven’t had a period, that’s worth bringing up with a doctor.
Your Body Needs a Certain Amount of Energy
Your reproductive system is, in biological terms, optional for survival. When your body senses it doesn’t have enough fuel, it redirects energy away from reproduction and toward keeping your heart beating and your brain functioning. This is one of the most common reasons a period is late to arrive or disappears after it has already started.
The threshold researchers have identified is about 30 kilocalories per kilogram of lean body mass per day. That number represents the energy left over after exercise. So if you eat 2,000 calories but burn 800 through intense training, what remains for your body’s basic functions may drop below the line your brain needs to keep your cycle running. This is especially common among competitive athletes, dancers, and anyone restricting calories heavily.
Body fat plays a direct role, too. Research from the Proceedings of the Nutrition Society found that a first period typically requires roughly 17% body fat, while maintaining regular cycles afterward requires closer to 22%. A well-nourished adult woman carries about 26 to 28% body fat on average. If your weight is significantly below what’s expected for your height and age, your body may be holding off on starting your cycle until conditions improve.
Stress Can Shut It Down
Emotional and psychological stress affects the same brain signals that control your cycle. The hypothalamus, a small region at the base of your brain, acts as the control center for hormone release. When it senses chronic stress, whether from school pressure, family problems, anxiety, or major life changes, it can slow or stop sending the signals your ovaries need to do their job. This is called functional hypothalamic amenorrhea, and it’s remarkably common in teens and young adults.
The tricky part is that stress, undereating, and overexercising often overlap. A student training hard for a sport while not eating enough and worrying about performance hits all three triggers at once. Addressing just one of those factors sometimes isn’t enough to bring a period back.
Hormonal Conditions That Delay Periods
PCOS
Polycystic ovary syndrome is one of the most common hormonal conditions in young women. In adolescents, diagnosis requires a combination of signs: excess hair growth on the face or body (hirsutism), severe acne, menstrual irregularities that persist at least two years after the first period, and elevated levels of androgens (sometimes called “male hormones,” though everyone produces them). Simply having cysts visible on an ultrasound, without these other symptoms, is not enough for a diagnosis in a teenager. Insulin resistance and obesity often accompany PCOS but aren’t part of the diagnostic criteria.
Thyroid Problems
Both an underactive and an overactive thyroid can disrupt your cycle. Thyroid hormones influence nearly every step of the reproductive process. They help regulate the brain signals that trigger ovulation, affect how your body handles estrogen and progesterone, and support the growth of the cells in your ovaries that mature eggs. When thyroid levels are off, ovulation can stall or stop entirely, and periods become irregular or disappear. A simple blood test for thyroid-stimulating hormone (TSH) can identify this, and treatment with thyroid medication often restores normal cycles.
High Prolactin Levels
Prolactin is the hormone that stimulates milk production after pregnancy. When levels are elevated outside of pregnancy, often due to a small benign growth on the pituitary gland or as a side effect of certain medications, it interferes with the hormones that drive your menstrual cycle. This is one of the six most common causes of amenorrhea identified by the American Society for Reproductive Medicine.
Medications That Can Interfere
Several types of medication can delay or stop periods by disrupting hormone balance. These include certain antipsychotic drugs, some antidepressants, blood pressure medications, chemotherapy and radiation therapy, and even some allergy medications. Hormonal birth control is another common cause: some methods are specifically designed to reduce or eliminate periods, and it can take several months after stopping them for your cycle to resume. If your period disappeared around the time you started a new medication, that connection is worth investigating.
When a Period Stops After Starting
If you’ve already had periods and they’ve stopped for three months or more (or six months if your cycles were always irregular), that’s secondary amenorrhea. The first thing to rule out is pregnancy, even if you think it’s unlikely. After that, the most common culprits are the same conditions listed above: PCOS, thyroid dysfunction, high prolactin, stress, and low body weight or energy availability.
It’s also worth knowing that irregular periods are extremely common in the first two years after your first period. Your body is still calibrating its hormone cycles, and skipping a month or having unpredictable timing during this window doesn’t necessarily signal a problem. Consistent irregularity beyond two years is when doctors start looking more closely.
What Testing Looks Like
If you see a doctor about a missing period, the evaluation is straightforward. The standard initial workup includes a pregnancy test and blood draws to check four key hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which drive ovulation; prolactin; and thyroid-stimulating hormone (TSH). These four tests together can point toward or rule out most common causes.
Depending on the results, your doctor may order a pelvic ultrasound to check for structural differences in the uterus or ovaries. In rare cases where a pituitary issue is suspected, an MRI of the head may be recommended. The blood work itself takes minutes, and results typically come back within a few days. Most causes of a missing period are highly treatable once identified.